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Temperature effects on the isomer’s stability ofvan der Waals clustersTo cite this article: Orlando Carrillo-Bohórquez et al 2017 J. Phys.: Conf. Ser. 875 042011

 

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Temperature effects on the isomer’s stability of van der Waals clusters

Orlando Carrillo-Bohórquez†, Álvaro Valdés†1, Rita Prosmiti∗2

† Departamento de Física, Universidad Nacional de Colombia, Calle 26, Cra 39, Edificio 404, Bogotá, Colombia∗ Institute of Fundamental Physics (IFF-CSIC), Serrano 123, 28006 Madrid, Spain

Synopsis Small van der Waals(vdW) complexes have serve as prototypes for investigating energy transfer mechanisms andmiscrosolvation processes. High-resolution spectroscopy techniques have provide valuable information, new aspects of thesesystems emerged from the interplay between theory and experiment, thus such systems are ideal for developing and testingtheoretical methods and computational approaches.

High-level ab initio electronic structure calcula-tions have shown that triatomic vdW clusters, formedby a dihalogen molecule and a noble gas atom,present double minima at both linear (L) and T-shaped (T) geometries in their ground electronicstates [1]. In the case of the He-dihalogen (homonu-clear) systems, such like HeBr2 and HeI2, the energydifferences between them counts only to few cm−1,while when nuclear quantum zero-point-energy ef-fects are taking into account the relative stabilityof the corresponding isomers is less than 1 cm−1.The existence and preferential stability of such iso-mers have been also supported by recent experi-mental studies from LIF and action spectra tech-niques [2]. However, discrepancies between theoret-ical predictions and even with experimental data ofdifferent sets of experiments have been reported, andthis arises the question of whether there is a simpleexplanation of the theoretical point of view on thestability order of such isomers. Although, a numberof experimental studies have detected different iso-mers of such vdW complexes in the supersonic ex-pansions, the mechanisms for their formation, andtheir relative populations within the expansion underconditions of low temperature (below 3 K) are stilluncertain and probably depend on the system. Thus,investigations on how thermodynamic factors (tem-perature) influence the isomers’ formation, and thustheir population in the supersonic beam expansion,could serve to improve deficiencies and shed light onour understanding of such processes.

As such a thermodynamic model was in-troduced [3] to compute the isomers’ popula-tion as a function of temperature T , ZL,T(T ) =ZL,T

vib (T )ZL,Trot (T ), where Zvib and Zrot the vibrational

and rotational partition functions, respectively. Rovi-brational quantum calculations for angular momen-tum values J up to 15 were carried out, all boundstates were assigned and then vibrational and rota-tional partition functions were computed. On the ba-sis of the present results, [3] the relative populations

of the linear and T-shaped conformers show a strongdependence on the temperature for both HeBr2 andHeI2 vdW complexes (see Figure 1). We found thatthe linear HeBr2 isomer is energetically more sta-ble than the T-shaped ones by 1.14 cm−1 at T=0 K,whereas conversion from linear to T-shaped com-plexes was observed at temperatures above 2.87 K.In turn, for the HeI2 system the population inversionoccurs at 1.04 K, with the energy difference betweenthem being just 0.21 cm−1. Interestingly, for theHeBr2 a nice accord between experiment and theoryhas been achieved, while the HeI2 case is still underconsideration, pushing for further improvements inthe both theoretical and experimental directions.

0 1 2 3 4T (K)

0

0,2

0,4

0,6

0,8

1

ZL

,T / Z

TO

T

HeBr2(L)

HeBr2(T)

HeI2(L)

HeI2(T)

T =1.04 K c

Linear

T−shaped

c T =2.87 K

Figure 1. Population ratio of linear and T-shaped iso-mers as a function of temperature. The two populationsbecome equal at TC=1.04 and 2.87 K for the HeBr2

(solid lines) and HeI2 (dashed lines), respectively. Inthe inset panels the density distribution of each, linearand T-shaped, vibrational state are displayed in the ZX-plane together with the underlying potential energy sur-face.

References

[1] L. García-Gutierrez et al. 2009 J. Phys. Chem. A 1135754

[2] S.E. Ray et al. 2006 J. Chem. Phys. 125 164314

[3] O. Carrillo-Bohórquez et al. 2016 J. Phys. Chem. A120 9458; 2017 RSC Adv. submitted

1E-mail: avaldesl@unal.edu.co2E-mail: rita@iff.csic.es

ICPEAC2017 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 875 (2017) 042011 doi:10.1088/1742-6596/875/5/042011

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